A federal appeals panel Wednesday refused to order closure of shipping locks on Chicago-area waterways to prevent Asian carp from invading the Great Lakes, but warned that it might reconsider if the government lags in its efforts to block the dreaded fish's path.

The 7th U.S. Circuit Court of Appeals in Chicago became the latest of several courts to turn down a plea from five states for immediate shutdown of the locks and a quicker timetable for other steps to halt the carp's northward march from the Mississippi River toward Lake Michigan, from which it could spread to the rest of the Great Lakes. The U.S. Supreme Court twice has rejected the request from Michigan, Minnesota, Ohio, Pennsylvania and Wisconsin. Federal District Judge Robert Dow in Chicago did likewise last December.

The states have a pending lawsuit against the U.S. Army Corps of Engineers and the Chicago water reclamation district that calls for permanently severing a century-old, man-made link between the Mississippi and Great Lakes drainage basins. They had sought a court order to close the locks, which could provide a pathway to Lake Michigan for the carp, while their suit works through the courts. With the appeals panel's ruling, that prospect appears remote.

Significantly, however, the three-judge panel disagreed with Dow's conclusion that the states appeared to have little chance of succeeding in their lawsuit. Dow had acknowledged a carp invasion could harm the Great Lakes but said the states hadn't shown it was likely or imminent. He said flooding and economic damage from closing the locks would do more damage to Chicago-area businesses and homeowners than "the more remote harm associated with the possibility" of Asian carp becoming established in the lakes.

In their opinion, Judges Daniel Manion, Diane Wood and Ann Claire Williams said they were "less sanguine about the prospects of keeping the carp at bay."

"In our view, the plaintiffs presented enough evidence at this preliminary stage of the case to establish a good or perhaps even a substantial likelihood of harm — that is, a non-trivial chance that the carp will invade Lake Michigan in numbers great enough to constitute a public nuisance," the judges said.

The Obama administration has pledged more than $120 million for a wide-ranging strategy to keep the carp out of the lakes. Because of those efforts, requiring immediate actions such as lock closure "would only get in the way," the judges said. "We stress, however, that if the agencies slip into somnolence or if the record reveals new information at the permanent injunction stage, this conclusion can be revisited."

The five states could take their request to the full 7th Circuit court or try again before the Supreme Court. No decision has been made, said Joy Yearout, spokeswoman for Michigan Attorney General Bill Schuette.

"This ruling is disappointing, but (Schuette) is going to continue the fight for long-term separation and that can happen through our current lawsuit," Yearout said.

Wisconsin Attorney General J.B. Van Hollen said despite its ruling, the appeals panel had "found significant merit in our claims."

"I believe the aggressive actions taken by Wisconsin and the other plaintiff states have forced the federal government to take this issue more seriously, and the Seventh Circuit acknowledges as much by suggesting that preliminary relief may very well be granted in the future, should the federal government's efforts wane," Van Hollen said.

Bighead and silver carp, both Asian imports, escaped from fish farms and sewage treatment plants in the Deep South in the early 1970s and have been migrating northward in the Mississippi and tributary rivers.

They have reached the Chicago Sanitary and Ship Canal, built in the late 1800s to flush the city's wastewater south toward the Mississippi.

A three-part electric barrier was constructed on the canal over the past decade to prevent invasive species from crossing between the two watersheds. But more than 80 water samples taken above the barrier have tested positive for DNA from silver or bighead carp, raising fears that at least some are getting through. Just one live Asian carp has been found beyond the barrier.

The Army Corps of Engineers is studying options for long-term prevention of species migration between the two drainage basins. Its report is due for completion in 2015.

The states' lawsuit calls for speeding up the study, along with stepping up measures such as poisoning waters above the barrier, spreading nets and erecting physical barriers where necessary.

Federal agencies say they have an effective program under way, including analyzing the electric barrier's effectiveness and monitoring the Chicago waterways.

When a series of water samples turned up carp DNA in Lake Calumet on the city's south side this summer, officials ordered an intensive netting and fish shocking effort. Yet again, they turned up no Asian carp.